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Smallest planet is nearly Earth-sized

Rocky Kepler-10b (at bottom in this illustration) is too close to its host star to support life as we know it

(Image: NASA)

Astronomers have found the smallest planet outside our solar system yet. The alien world is just 1.4 times as wide as Earth, but it is far too hot to host life as we know it.

NASA’s Kepler space telescope detected the planet, called Kepler-10b, indirectly by observing how it regularly dimmed its parent star when it passed between the star and Earth.

The amount of dimming indicates that the planet is 1.4 times the width of Earth, making it smaller than the previous record holder, COROT-7b, which is about 1.7 times Earth’s size.

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The team used ground-based telescopes to observe the wobble that the planet gravitationally induces in its parent star. That revealed that the planet is 4.6 times as massive as Earth.

Rocky world

Its density is about 8.8 times that of water, meaning it must be made mostly of rock and metal, like Earth.

COROT-7b may well be a rocky world, too. It has the same density as Earth – about 5.5 times that of water. But that measurement is less certain, so COROT-7b could instead be much less dense, with up to 50 per cent of its mass made up by water ice, says Kepler deputy science team leader leader Natalie Batalha of NASA’s Ames Research Center in Moffett Field, California.

There is less wiggle room in the density of Kepler-10b, she says.

The planet’s star is very similar in size and mass to the sun. But the planet orbits it at less than 4 per cent of Mercury’s distance from the sun, making the planet’s surface way too hot to support life as we know it. Intense radiation from the star probably also prevents the planet from holding onto an atmosphere.

Molten oceans

One side of the planet is likely permanently facing the star, heating it to around 1400 °C, “far in excess of the temperature of typical lava flows on Earth”, says Batalha. “Along that day side, we expect to find oceans of molten material.”

Planet hunter Geoff Marcy of the University of California in Berkeley, who was not involved in the discovery, said it is “the first definitive rocky planet ever found” outside the solar system.

Because it is a major milestone on the way to finding planets like Earth that could host life, Marcy said Kepler-10b “will be marked as among the most profound scientific discoveries in human history”.

Batalha announced the result at a press conference in Seattle, Washington, at a meeting of the American Astronomical Society.